The present invention relates to a method of manufacturing a protective leading edge strip for an aerofoil, such as a vane or blade. The invention is concerned particularly with a thermoplastic protective leading edge strip for a composite aerofoil vane.
Within aero engines the leading edges of rotating and stationary aerofoils are often subjected to high-levels of erosion and impact loading. In particular fan blades and guide vanes endure harsh abrading environments including dust, sand, ice and water as well as occasional impacts from foreign bodies such as birds and other debris. Therefore the leading edges are often reinforced to make them more resilient to these environments.
Conventionally, with metallic aerofoils the choice of metal may be sufficient to ensure the appropriate resistance to the harsh environment, or else a surface coating may be added to increase the resistance. However when considering composite technologies for fan blades and guide vanes, the composite material alone is not sufficient to withstand common levels of erosion or of impacts. Accordingly, if no extra protection is afforded to the composite blade at its leading edge, damage can propagate into the more structural parts of the aerofoil. Because of this, solutions have been proposed which include wrapping pieces of metal around the leading edges. This gives some protection of erosion and also give the possibility to dress back the leading edge. It also provides protection against impact from foreign bodies. Such an approach has been widely adopted in the field of aero engines.
However, when using a metallic sheath on the leading edge of a composite aerofoil there is a need to apply separate surface treatments to both the sheath and the aerofoil and then bond the two together, which requires an extra production step. Furthermore, as the metallic sheaths are not generally structural components of the aerofoil they add weight without adding structural performance.
An additional problem arises in that air worthiness regulations specify that any separate, or separable, component must be contained within the engine and must not endanger the aircraft or any ground equipment. Therefore it is necessary to take steps to contain any metallic leading edge which has the possibility to detach during high-energy impact events and become released. This is an especially important issue when used on rotating components such as fan blades. If a metallic leading edge is released from a fan blade it can become effectively a high-energy spear which, if not contained, can pose a serious threat to the aircraft. Containing metallic components of this kind can necessitate an increase in both the cost and the weight of the structures required to contain them.
Currently some composite vanes have polyaryl ether ether ketone or polyetheretherketone (PEEK) thermoplastic erosion systems. The method of manufacture is to use both heat and pressure to form a PEEK sheet onto some glass fabric over a moulding tool to make a PEEK leading edge strip. The mould tool itself represents the first 30 mm or so of the aerofoil, and thus defines the leading edge strip component itself.
Once this leading edge strip has been formed it is placed in the resin transfer mould (RTM) tool cavity along with the remaining materials that go to make up the composite vane, the other materials comprising mainly carbon fibre 3D woven preforms and 2D carbon fibre fabrics.
Once layup is complete the mould cavity is injected and filled with resin which is then cured.
During this process the resin that fills the mould micro-fills the glass fabric on the PEEK leading edge strip component. This interaction between the glass fabric and the resin provides a bond interface between the PEEK material and the vane itself.
The manufacture of the PEEK leading edge strip is done in a compression mould tool. The glass fabric and PEEK sheet are laid over the tool and pressure is then applied to pre-clamp the tool and materials. The tool is then placed in an oven to ensure that the temperature is controlled to within very tight limits all over the surface of the tool. Once the PEEK has reached the melt temperature it partly flows into the glass fabric.
Temperature and pressure variance across the tool surface directly affects the amount of PEEK that flows into the glass fabric and therefore can result in a variation in the strength of the bond between the two materials. The difference in the co-efficient of thermal expansion (CTE) of the materials also results in “spring-back” therefore affecting the form of the part, as well as causing inherent stress between the two materials which leads to potentially weak areas in the bond interface.
An alternative previously considered method is to use just a PEEK fabric by itself. However when this is used the woven architecture of the fabric opens up as it goes around the edge, and this causes a rough surface which is not deemed aerodynamically acceptable. Furthermore because the fibres have opened up around the edge the material becomes structurally weakened in this crucial area.